The invention relates to an apparatus for the zone pulling of monocrystal rods, which consists of a hermetically sealed housing having rod holders disposed one over the other within the said housing for rotation and vertical movement, and a fixed high-frequency heating system disposed between them, and having a transport slide with sliding guide means and spindle drive for the independent vertical movement of each rod holder, and having a drive for the rotation of each rod holder.
An apparatus of the above-described kind is disclosed in German Auslegeschrift No. 15 19 871. In it the slide guides and the spindle drives for the rod holders are disposed outside of the housing, and the shafts at whose extremities the rod holders extend through the wall of the housing. The heating system in the form of a high-frequency coil is disposed fixedly approximately in the center of the housing.
In this type of design, the housing fundamentally must first have a slightly greater height than twice the maximum length of the rod which is to be remelted without the use of a crucible, or the length of of the monocrystal which is to be produced, as the case may be, since virtually the entire length of the rod has to pass through the heating system. To the height of the housing must be added the lengths of the two rod holder drive shafts, since they have to perform a travel of that length outside of the housing. Also to be added to the total overall height are the intervals necessitated by the indispensable mountings and guides and the housing as well as the drive motors. Of course, the length of the spindle drives outside of the housing must correspond also to the vertical movements of the above-described drive parts and rod holders. The structural height of such known apparatus therefore generally amounts to not less than about 5 to 6 times the useful crystal length, which is less than the length of the raw monocrystal, since the transitional taper and other portions have to be discarded. In one practical embodiment, the total height of the apparatus is 8.0 m, whereas a useful crystal length of no more than 1.4 m can be achieved. Such a structural height, however, is intolerable for most applications, since only rarely is production space available with this amount of overhead clearance.
It is also known to reduce the structural height of zone pulling installations by a different principle of design. In this case the rod which is to be remelted is held at substantially the same height within the housing, while only one of the two rod holders has to perform a slight vertical movement corresponding to the draw-press ratio to be maintained in the production of the crystal. For the performance of the necessary relative movement between the rod and the heating apparatus, however, the high-frequency coil must be moved vertically in this case. This, however, leads to special problems with regard to the supply of power to the high-frequency coil. A cable connection to the high-frequency generator is not only disadvantageous for safety of operation, but it also results in considerable power losses due to the length of the cable. Any kind of rigid connecting of the high-frequency coil to the oscillating circuit makes it necessary to locate the latter within the housing. Thus an undesirably large pot diameter has to be accepted, while the problem of the moving power supply is still unsolved. In addition, an oscillator circuit for an output power of 50 kW weighs on the order of about 250 kg, so that, in the case of vertical movement of the oscillator circuit, an appropriately stable guidance system has to be provided along with a sufficiently powerful drive. At the same time, it is exceedingly difficult to assure the necessary freedom from vibration. Consequently, this kind of design principle of kind utilized heretofore only in the case of relatively small monocrystals.